1. Field of the Invention
The present invention relates to a beam irradiation apparatus which irradiates a target area with a laser beam, particularly to a beam irradiation apparatus mounted on so-called laser radar which detects existence of an obstacle within the target area and a distance to the obstacle based on reflected light in irradiating the target area with the laser beam.
2. Description of the Related Art
Recently, a laser radar is mounted on an automobile for family use or the like in order to enhance safety during running. In the laser radar, the front in the running direction is irradiated with a laser beam, and existence of an obstacle in a target area and a distance to the obstacle is detected based on the state of the reflected light. Usually, in the laser radar, the target area is scanned with the laser beam, the existence of the obstacle at each scanning position is detected based on the existence of the light reflected from each scanning position, and the distance to the obstacle at each scanning position is detected based on a necessary time between reflected light acceptance timing and laser beam irradiation timing at the scanning position.
In order to enhance detection accuracy of the laser radar, it is necessary that the target area be properly scanned with the laser beam, and it is necessary that each scanning position of the laser beam be properly detected. A scanning mechanism in which a polygon mirror is used and a lens drive type scanning mechanism in which a scanning lens is two-dimensionally driven have been known as the laser beam scanning mechanism.
In the scanning mechanism in which the polygon mirror is used, a side face of the polygon mirror is irradiated with the laser beam while the polygon mirror is rotated, thereby performing the laser beam scanning. The polygon mirror has a polygon in section, and the mirror is formed in each side face. When the side face of the polygon mirror is irradiated with the laser beam while the polygon mirror is rotated, an incident angle of the laser beam is changed with respect to each side face, and whereby the light beam reflected from the side face is deflected in a polygon mirror rotating direction.
However, in the scanning mechanism, a high-accuracy plane forming technique and application of high-performance motor are necessary because plane accuracy of the mirror surface and a mirror rotating state have a large influence on scanning performance. Additionally, because the mirror surface has a small angle change with respect to a laser beam axis, unfortunately a swing angle of the laser beam is not increased too large.
On the other hand, in the scanning mechanism in which a lens actuator is used, because the scanning is performed by lens drive, the two-dimensional scanning operation can be realized with a relatively simple configuration. However, because the lens is supported by a plate spring or a wire, unfortunately the lens actuator is susceptible to vibration from the outside. When the rigidity of the plate spring or wire is increased to suppress the vibration, a response characteristic is decreased during the scanning operation. Additionally, because the laser beam is incident to an eccentric position with respect to a lens optical axis during the scanning operation, a distortion is generated in an intensity distribution of the laser beam.
The inventor studies a mirror drive type actuator, in which the response characteristic can be enhanced with a simple configuration while the distortion of the laser beam can be suppressed. In the mirror drive type actuator, the mirror is supported in a biaxial drive manner and the mirror is rotated about each drive axis by an electromagnetic drive force between a coil and a magnet. The laser beam is incident to the mirror from an oblique direction, and the mirror is two-dimensionally driven about each drive axis. Therefore, the target area is two-dimensionally scanned with laser beam reflected from the mirror.